1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic apparatus, an electrostatic recording apparatus, etc., and more particularly to a color image forming apparatus having a plurality of developers.
2. Description of the Related Art
FIG. 12 is a cross-sectional view of an example of a full-color image forming apparatus having a plurality of developers. In FIG. 12, a photosensitive drum 1, which is a latent image carrier, is provided approximately in the center of the main body of apparatus, a supporting member 7 supports a plurality of developers 4a, 4b, 4c, 4d on the right side with respect to the drum 1, and a transfer drum 5 is set on the left side with respect to the photosensitive drum 1. The transfer drum 5 performs functions of carrying a transfer sheet (not shown) and transferring an image on the photosensitive drum 1 onto the transfer sheet. In the upper region inside the apparatus body there is an exposure device consisting of a laser diode 11, a polygon mirror 13 rotatably driven by a high-speed motor 12, a lens 14, and a return mirror 15.
The photosensitive drum 1 is rotated in the direction of the arrow and uniformly charged by a charging means 2. An optical image through an optical path 16 is projected onto the photosensitive drum 1 to form an electrostatic latent image thereon. This latent image is developed into a visible image, i.e., into a toner image by any one of the developers (4a, 4b, 4c, 4d) accommodating color developing agents, for example, of yellow (Y), magenta (M), cyan (C), and black (Bk), respectively.
On the other hand, the transfer sheet is supplied from inside a transfer sheet cassette 17 onto the surface of the transfer drum 5 by a pickup roller 18 in synchronization with the image on the photosensitive drum 1. Keeping the supplied transfer sheet gripped by a gripper 19 and adhered to the transfer drum 5 by an adhering roller 20, the transfer sheet is guided to a position where it opposes the photosensitive drum 1. The toner image on the photosensitive drum 1 is transferred in a superimposed manner onto the transfer sheet wound around the transfer drum 5 with an electric field applied between the photosensitive drum 1 and the transfer drum 5.
Describing in further detail, for example, the developer 4a accommodating the yellow (Y) developing agent first visualizes a first electrostatic latent image formed on the photosensitive drum 1 by exposure based on a first color image signal, and thereafter the toner image is transferred onto the transfer sheet held on the transfer drum 5. Subsequently, residual toner on the photosensitive drum 1 is cleaned by a cleaning device 6 such as a fur brush, a blade means, etc. Then a second color electrostatic latent image is formed on the photosensitive drum 1 by exposure based on a second color image signal, for example, by the developer 4b having the magenta (M) developing agent. Thereafter, the toner image is transferred onto the transfer sheet on the transfer drum 5, and superimposed on the first color, yellow, a visible image having been transferred.
Next, repeating the same steps as above, a toner image of a third color, for example, of cyan (C) and a toner image of a fourth color, for example, of black (Bk), are successively transferred in a superimposed manner onto the transfer sheet on the transfer drum 5. After that, the transfer sheet is separated from the transfer drum 5 by a separating claw 8, and the resultant toner image is fused, mixed, and fixed by a fixing device 9 to obtain a full-color permanent image.
After separating the transfer sheet, the transfer drum 5 is cleaned by a transfer roller cleaning device 22 such as a fur brush, a web, etc. to remove toner deposited on the surface, and an eliminator roller 21 eliminates charge on the transfer drum 5 to electrically initialize the drum. The eliminator roller 21 is also used for eliminating charge given in transferring the toner image from the photosensitive drum 1 onto the transfer drum 5.
The above color image forming apparatus would fail to achieve correct color tone if the image density changed depending upon various conditions such as operating temperature-humidity environment, the number of prints, etc. Thus, the conventional apparatus is arranged so that a toner image for detection of density is formed on the photosensitive drum or on the transfer drum, the density of the image is detected by density detecting means, and the detection result is fed back to control the exposure amount, development bias, etc., thereby obtaining stable and excellent images.
However, the color image forming apparatus arranged to have the means for forming the toner image for detection of density on the photosensitive drum or on the transfer drum, for detecting the density thereof, and for feeding the detection result back to control the exposure amount, development bias, etc. has a drawback of increased print cost, because it is arranged to form detection-purpose toner images of respective colors of yellow, magenta, cyan, and black, which requires a lot of toner of respective colors, thus consuming a lot of toner for a purpose other than printing on the transfer sheet. Another drawback is a slowdown of printing speed, because time is consumed for producing a number of toner images and for detecting the density thereof. A further problem is an increase in the amount of waste toner after cleaning.